The present invention relates to a fluorescent lamp having a reflective layer. More particularly, the present invention relates to a fluorescent lamp in which phosphor layers are partially laminated and a substantially reflective function is exerted by utilizing the difference of the thickness between the phosphor layers.
Ordinary fluorescent lamps are formed by coating a single layer of a phosphor on the entire surface of the inner wall of a glass tube so that the thickness is substantially uniform. In such ordinary fluorescent lamps, distribution of irradiance substantially equal in the radial direction from the center of the glass tube is obtained. However, in practical application, the luminous flux in the direction of main irradiance, that is, in a specific radial direction from the center of the glass tube, is mainly used.
For this purpose, a fluorescent lamp having a reflective layer is suitable. As typical instance of the fluorescent lamp of this type, there can be mentioned a fluorescent lamp in which a reflective layer of titanium oxide or the like is formed on the inner wall of a glass tube on the side opposite to the direction of main irradiance and a phosphor layer is coated on the entire inner face of the glass tube. In this fluorescent lamp, the luminous flux generated in the glass tube is convergently radiated from the light transmitting face in the lower portion of the glass tube to obtain a strong illuminance just below the lamp. We already proposed a fluorescent lamp having a sectional structure shown in FIG. 1 (see U.S. patent application Ser. No. 089,744 filed on Oct. 31, 1979). FIG. 1 illustrates the structure of the section of a straight or circular fluorescent lamp of the above-mentioned reflective layer type taken vertically to the central axis of a glass tube 1. In this lamp, a first phosphor layer 2 is formed on the entire face of the inner wall of the glass tube and a second phosphor layer 3 is formed on a part of the first phosphor layer 2, for example, along the axial direction of the glass tube 1 at a coating angle .theta. of 180.degree.. In this fluorescent lamp, on the inner wall of the glass tube 1 there is formed a phosphor layer comprising a thick portion where the phosphor layers 2 and 3 are laminated and a thin portion consisting solely of the phosphor layer 2. The thick portion has not only the function as the reflective coating but also the function of light transmission. Namely, this fluorescent lamp of the reflective layer type is characterized in that the illuminance in the direction of main irradiance is increased and simultaneously, a certain illuminance is given also in the direction opposite to the direction of main irradiance.
Ordinarily, a laminated phosphor layer structure such as mentioned above is formed by first coating a phosphor uniformly on the entire face of the inner wall of the glass tube 2 and drying the coating to form a phosphor layer 2, then coating a phosphor uniformly on a part of the phosphor layer 2, for example, in a region of a coating angle .theta. of 180.degree. and drying the coating to form a phosphor layer 3, and finally baking the phosphor layers 2 and 3 simultaneously.
A combination of phosphors having the same emission spectrum of light are ordinarily used for the fluorescent lamp of the reflective layer type including laminated phosphor layers. However, even if phosphors differing in the emission spectrum of light are used in combination, it is possible to increase the illuminance in the direction of main irradiance and improve the color rendering properties.
In the conventional fluorescent lamp having the entire face of the inner wall of the glass tube 1 uniformly coated with phosphors, phosphors having a particle size providing a highest capacity are used as the phosphors of the layers 2 and 3, and halo-phosphate phosphors having an average particle size of 10 to 14 .mu.m are used most popularly.
In the fluorescent lamp having the above-mentioned laminated structure of phosphor layers, the effect of improving the illuminance in the direction of main irradiance is only such that the illuminance is increased by 15 to 20% over the illuminance of the conventional fluorescent lamp, and this effect of increasing the illuminance is still insufficient as compared with the effect attained by the conventional fluorescent lamp of the reflective layer type formed by using a reflective material such as titanium oxide.